1. The Field of the Invention
Embodiments of the present invention relate to display and pixel architectures suitable for the manufacture of bi-stable, reflective, magneto optical displays. These displays are suitable for a multitude of applications, especially cost-sensitive, large-format outdoor signage applications. These new display architectures have ultra-low power consumption, are light weight, thin and leverage existing low cost industrial manufacturing processes.
2. The Relevant Technology
The principles of the present invention relate to a new type of reflective, bi-stable, magneto optical display especially well suited for large format, outdoor and high brightness applications. The core components of this invention can be manufactured using low cost, industrial manufacturing processes.
Traditional flat panel display technologies like plasma, liquid crystal and organic light emitting diode displays have proven unsuccessful for use in large format signage applications. These display technologies do not offer sufficient brightness and contrast when exposed to direct sunlight, where ambient light can reach 100,000 lux, or other high brightness environments where ambient light is greater then 1,000 lux. In addition they are not cost effective or reliable when put in large format, outdoor environments.
Today, large panel arrays of discrete light emitting diodes (LEDs) have been developed to fill the need of the digital signage market. These indoor and outdoor digital signage devices are very expensive products with a high number of discrete components and high power requirements. They must use a high number of densely packed high-brightness LEDs to emit enough light to compete directly with sunlight in outdoor and high brightness applications. Direct sunlight can be up to 100,000 lux. A simple one color, 4 line×20 character LED display will typically use approximately 2,800 discrete LEDs. An LED “Jumbotron” large format, color video screen may require greater than 1 million discrete components.
Another feature the sign industry is looking for is reflective technology in place of light emitting. With reflective displays, direct sunlight is used during the day for illumination. At night, illumination requirements are far lower so the power used by a reflective display even with night lighting will be much lower. Another benefit is bi-stability. This is a display that maintains the image without continuous power once the image is written to the display. This enables very low power consumption in applications such as information displays and signage. This is because many signs will not require information to be changed frequently. For example, a gas price sign may only be updated once a day or a clock only changes one digit every 60 seconds. Bi-stable displays can be ultra-low power making it possible for them to be powered by small batteries or solar cells. This may eliminate the need for costly wiring especially where the sign may not be near utilities. Finally, light weight and thin digital signs are desirable because they reduce the cost of installation and the support structure required. The invention described herein has all of these features: reflective, bistable, low power, thin and light weight.
Prior to LED signs, starting in the early 1900s, there were various electronic signage technologies that used electromagnetic actuators (Naylor U.S. Pat. No. 1,191,023, Taylor U.S. Pat. No. 3,140,553 and Browne U.S. Pat. No. 4,577,427). These discrete actuators were generally variations of small electromagnetic coils or motors with a reflective mechanical flap apparatus. They were broadly used and a small number of these device are still in use today for specific outdoor signage applications like score boards and transit signs. However, they have become obsolete with the advent of LEDs because they are very expensive to construct, suffer from reliability issues and are limited in resolution due to the size of the discrete mechanical assemblies needed for each pixel.
Development continued in the use of magnetic actuators for displays by Weiacht (U.S. Pat. No. 6,510,632) where magnets were attached to large flaps or “flags” that could be rotated 180 degrees in forming a signage character. This type of technology was continued with the patents of Fischer et al and others (U.S. Pat. Nos. 3,936,818, 6,603,458) that again discussed the use of magnetically driven flaps as individual display pixels. These devices are mounted on a mechanical axis and only one magneto-optic “flag” is used in each individual device.
While magnetics itself and the study of magnetic materials is an old discipline that dates back centuries and is still studied heavily today (see Spaldin, N., Magnetic Materials Fundamentals and Device Applications, Cambridge Univ. Press, 2003.; Kittel, C. Introduction to Solid State Physics, Wiley, 1996.), few attempts have been made in recent years to develop a new type of reflective magnetic-based display technology. In 1898 and 1928 Poulsen and Pfleumer, respectively, leveraging magnetic phenomena, developed the use of magnetics in recording media. In the 1960's magnetic materials were used to store memory, first by Forrester et al and others (U.S. Pat. Nos. 2,736,880, 2,667,542, 2,708,722). Magnetic core memory, pioneered by Olsen and others (U.S. Pat. Nos. 3,161,861, 4,161,037, 4,464,752) has seen significant attention for decades due to its implications on the computing industry but for a number of reasons was replaced by silicon-based memory devices.
The primary work in the use of magnetics in displays outside of the older “flap” technologies utilizes bi-colored magnetic particles instead of mechanical flaps. Here, the use of smaller discrete magnetic spheres has been envisioned as a display design by Magnavox (Lee, L. IEEE Transactions on Electron Devices, ED-22, P758) and more recently by Katsuragawa et al. (Japan Patent Publication 2002-006346) and to a lesser extent by Masatori (Japan Patent Publication 08-197891). This prior art deals primarily with the use of small magnetic particles which respond to external magnetic fields by rotating. The rotation of these magnetic particles is in response to an external field that is required to prevent the particles from returning to a lower potential energy state. These magnetic spheres are magnetized along the center axis. The north and south poles of the spheres are coated with different colors. A magnetic field is then applied that rotates the particles between the two color states. This method of making an electromagnetic display has several problems which will now be discussed.
A first problem is an external magnetic field needs to be sustained. Without an ongoing electromagnetic field, the particles will align to each other in a low energy, grey state. Two methods to sustain the magnetic field are: 1) Use constant current to the electromagnetic coil—the issue here is constant power consumption and it requires much higher cost control electronics. 2) Add a layer of ferrite/“writable” magnetic material. This layer must sustain a sufficient magnetic field after it has been “written” by an electromagnetic write pulse. One issue here is that the added layer is a complex magnetic material that needs to be developed. Also the strength of the magnetic field and stability of the “writable” magnetic state is complex. There is significant added cost of manufacturing with this more complex structure. Finally, the energy required to create a sufficient B magnetic field inside the material to “write” a sustainable magnetic state can be considerable.
A second problem is crosstalk between pixels. The flux patterns created by two pixels of different states will create curved flux lines between their poles. The transition from one pixel's color state (North) to the second pixel's color state (South) will create flux patterns that are displayed as artifacts between pixels that will be viewed as a “grey” zone. Creating distinct separation between pixels without crosstalk is a major problem. Another form of crosstalk can occur when the magnetic field of one pixel “overpowers” a neighboring pixel and reverse its state during the write process.
A third problem is low contrast. Spherical particles, if placed in a single layer, will have limited contrast because they do not have full coverage of the display plane. If multiple layers of magnetic spheres are used, this increases their interference with each other, making it more difficult to control their states and thus requiring a stronger external magnetic field.
A fourth problem is magnetic materials. Development of new materials of construction and methods of fabrication are needed. The materials and methods must create low cost, light weight, environmentally stable, permanent magneto optical elements with accurate alignment of the magnetic field to the color planes.
A fifth problem is resolution limitations. This is because the domains of permanent magnetic particles lose stability when they are isolated or particles are made too small (for example, below 100 microns a drop in stability in magnetic particles starts to be observed depending on the specific material). In addition, electromagnetic coil assemblies become less efficient when the size is below 1 mm due to heat losses in the wire windings from the increased resistance of the smaller conductors. Also, in order for low cost, industrial manufacturing processes to replace high cost “clean room” processes, the manufacturing tolerances need to be in the range of +/−25 microns or larger to enable ease of manufacturing.
A sixth problem is design of a low cost electronic backplane. The highest cost processes in flat panel displays is the manufacturing of active matrix backplane layers that control the display. Most liquid crystal (LCD), organic light emitting diode (OLED) and even the latest electronic-paper (ePaper) displays utilize an array of thin film transistors (TFTs) that are manufactured over the entire back of the display to control the image. The cost of this backplane can equal 50% of the entire process costs of the display panel. A low cost backplane alternative is needed for large format signage applications.
The principles of the present invention discussed herein disclosure a significantly different architecture for producing a particle-based electromagnetic flat panel display. It is a new approach designed primarily for, although not limited to, large format and/or lower resolution applications (with pixel sizes of 1 millimeter or greater). This invention utilizes magnetics and electromagnetics to solve the problems described above.